Translation:ये मेरी माँ हैं।
Shouldn't it be यह rather than ये, as mother is singular?
ये is used here to show respect.
And is that why it is not है but हैं ?
Yes. 1st person हम and (in this example) 3rd person ये/वे/आप use हैं. In Hinglish, you'd type है like "hai" and हैं like "hain" to make it clear for anyone reading.